1. Field of the Invention
This invention relates to the liquefaction of coal or similar liquefiable carbonaceous solids and is particularly concerned with H-donor solvent liquefaction processes in which recycle solvents are recovered from liquids produced by the treatment of carbonaceous solids with molecular H and a donor solvent. The principal object of such direct coal liquefaction processes, sometimes referred to as solvent refined coal processes, is to stabilize molecular fragments generated by thermal degradable coal. This is achieved by "capping" the coal-derived free radicals with hydrogen. It requires the hydrogen be provided at a rate and in a form compatible with the demand established by the thermal reaction of the coal. Although the alternate source of H.sub.2 is hydrogen gas, the most efficient route for hydrogen transfer is by way of the liquefaction solvent. Hydrogen is supplied most readily from hydrogen donor compounds which may be process-derived and replenished by catalytic hydrogenation either within the liquefaction zone or in a separate zone.
2. Description of the Prior Art
Conventional schemes for the liquefaction of coal via solvent refining involve recycling a process-derived solvent boiling in the range from about 400.degree.-850.degree. F. to the coal dissolver. For example, U.S. Pat. No. 3,232,861 suggests hydrogenation of a deashed coal extract and recovering recycle solvent from the hydrogenated extract. Also see U.S. Pat. No. 3,523,886. Schemes wherein solvent boiling range material is separated from liquefaction zone effluent and is either hydrogenated in a separate catalytic solvent hydrogenation zone or returned directly to the liquefaction zone to be hydrogenated "in situ" have also been proposed. See U.S. Pat. Nos. 4,022,680; 4,049,536; 4,049,537; 4,051,012; and 4,123,347. U.S. Pat. No. 4,048,054 teaches a process wherein liquids obtained from vacuum atmospheric distillation and boiling in the range from 250.degree. to 850.degree. or 1000.degree. F. are hydrogenated in one solvent hydrogenation unit. Heavier recycle solvent materials boiling in the range from 700.degree.-1000.degree. F. are recovered by vacuum distillation. Hydrogenation of solvent materials may be catalyzed either by commercial hydroconversion catalysts or by the mineral matter in accumulated, liquefaction-reactor solids.
Copending U.S. Patent Application Ser. No. 055,948, filed July 9, 1979, now abandoned discloses novel solvent materials which are capable of being thermally hydrogenated in the absence of hydrogenation catalysts and also have the ability of being dehydrogenated or of donating hydrogen atoms to free radicals resulting from the depolymerization of constituents in solid carbonaceous material. These solvent materials are described as polycyclic hydrogen transfer solvents having polarographic potentials less negative than phenanthrene but equal to or more negative than azapyrene. Exemplary of such materials are pyrene, fluoranthene, anthracene, benzanthracene, dibenzanthracene, perylene, coronene, their heteronitrogen analogs, quinoline, and lower alkyl analogs of the foregoing. To be distinguished from the hydrogen transfer solvents are the hydrogen donor solvents of conventional processes, e.g., tetraline, alkyl-substituted tetraline, naphthalene, etc. Although the hydrogen transfer solvent may be mixed with o other materials such as hydrogen donor solvents, the process taught excludes contact of a separated recycle solvent fraction with heterogenous hydrogenation catalysts.
Copending U.S. Patent Application Ser. No. 107,817, filed Dec. 28, 1979 now U.S. Pat. No. 4,292,164, discloses a process for recovering hydrogen transfer solvents from a coal liquefaction product. Briefly, this application teaches removal of light hydrocarbon components boiling below 200.degree. C. from the coal liquefaction zone product and extraction of the heavier liquid with a process-derived naphtha having an aromatic content of 20 wt.% or less. The fraction of the heavier coal liquid which is soluble in the naphtha is recovered for recycle to the liquefaction zone as the hydrogen transfer solvent. Again, the process disclosed excludes a separate catalytic hydrogenation zone and relies on in situ hydrogenation/dehydrogenation of the hydrogen transfer solvent to liquefy solid carbonaceous feed.
A problem which has caused substantial difficulty in developing an economical operation for solvent extracting coal has been the inability to facilitate the transfer of sufficient hydrogen into the hydrogen-deficient coal during the extraction step. Hydrogen donor solvents have provided a partial solution to this problem. However, the amount of hydrogen which hydrogen donor solvents can typically transfer into the coal is quite limited, and, as noted above, a further hydrogenation treatment subsequent to the extraction operation is generally necessary when using them to extract the coal. The discovery of hydrogen transfer solvents and their facility for thermal hydrogenation is an important one, but enhanced hydrogen transfer during coal liquefaction is still desirable.
Accordingly, a primary object of the present invention is an improved, process-derived recycle solvent. A related object is an improved method for the recovery and treatment of recycle solvents from the products of coal liquefaction. A further related object is an improved method for the liquefaction of carbonaceous material.
Other objects and advantages of the present invention will become apparent from the accompanying description and examples.